As described in PCT Patent Application Ser. No. US2004/036446 filed Nov. 2, 2004 entitled Dual Frequency Through-the-wall Motion Detection and Ranging Using Difference-Based Estimation Technique, invented by Paul Zemany and Eldon Sutphin, and as described in patent application Ser. No. 11/121,787 filed May 3, 2005, multiple dual frequency radars spaced about a building have been used to localize the position of a moving individual in the building so as to provide information about the whereabouts of personnel, whether it is for the purpose of rescue, such as for firefighters, EMTs and the like, or to pinpoint enemy personnel in a building.
The above patent applications, incorporated herein by reference and assigned to the assignee hereof, describe the use of a multi-tone set of radars that detect and define the range to a moving object behind a wall, with triangulation techniques used to detect and track the moving object as it moves within the building or behind a wall.
Multi-tone CW radars are used, in which the two tones are fairly close together, with it being possible by analyzing the returns from moving objects to determine the range of the object to the various radar transmitters. By using multiple transmitters one can triangulate to be able to pinpoint the individual within the structure from the outside of the structure.
While the systems described in the above-mentioned patent applications work very well to detect the presence of a moving object and to detect its range and in fact its location, various problems nonetheless exist due to a number of factors, not the least of which is the fact that the wall of an edifice of building has material that differently affects radar beams at different frequencies due to the frequency-dependent difference in attenuation. It will be appreciated that in synthetic aperture radars, the angular resolution is given by λ/d where λ is the wavelength and d is the length of the baseline on which the measurement is to be made. However, resolution is critically dependent on pulse shape, and with pulse shape altered by wall materials, image resolution is adversely affected. There is therefore a requirement to take into account wall density and material to improve image accuracy.
Note that with both range and angle information, 2D representation consisting of range and azimuth pixels can be formed. Note also that if the baseline has both horizontal and vertical extent, 3D representation is possible. In this case each cell represents range, azimuth and elevation. However, without correction these representations will be fuzzy due to the alteration of the pulse shape as the pulses pass through the wall.
In general, frequency bandwidth provides range resolution and the baseline geometry provides angle resolution. The quality and/or details contained in the image depends on the bandwidth, baseline range, wall distortions, wall uniformity and wall absorption. In addition, the quality of the image depends on the position accuracy and velocity measurements.
Thus, due to the variability of wall transmissivity, the range measurements are coarse at best and it is only with difficulty that one can establish an accurate range to a moving object behind a wall.
If use could be made of the fact that the interaction of objects and RF energy is linear in the amplitude domain, one could potentially improve upon range accuracy by adding the amplitude response caused by a first signal to the amplitude response caused by a second signal to get the response caused by a signal that is the sum of both signals. As will be seen, because of this linearity in amplitude, it would be possible to use a set of closely-spaced, frequency-stepped CW measurements taken at widely different frequencies and closely-spaced points to provide improved resolution.
Moreover, all of the above multi-tone through-the-wall radars require object movement to obtain range. There are, however, returns from non-moving objects that complicate analysis of the returns. Thus artifacts from non-moving objects can corrupt the display of returns from moving objects.
Thus, while it is indeed useful to be able to ascertain whether or not a person is within a building due to the detection of their movement and while it is also useful to pinpoint, to the extent possible, their position and track using multiple multi-tone CW radars, there is nonetheless a requirement to obtain more precise measurements and more accurate tracks while at the same time distinguishing other artifacts in the room such as chairs, desk, tables and lamps, etc. These artifacts are in general non-moving so that theoretically one should be able to distinguish a moving object from one that is not.